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Current Sink for LEDs


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Hi everybody!

I want to design a project with some LEDs, something like a signboard and I need to use a current sink, which can drive up to approximately 500-600mA. I have thought of using a NPN transistor (for example 2N2222),  but I do not know what the exact circuit should look like. I have simply connected the E to the negative supply (ground), the C to the negative lead of the LED (of course the + of the LED is connected through a resistor to the positive supply) and the B to ground, but it doesn`t work at all!

Can anybody help me? Perhaps is there another solution, apart from using a transistor? (an IC maybe?)

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Hi Nick,

Why not use the LM317 or LM350?



Well... I haven`t understand what you mean Ante... :-[ I`m sorry...

How can the LM317 be used as a current sink?

Actually I mean something like the following circuit (except that instead of 1 I want to use more LEDs), where the ON or OFF state of the LED can be changed by the voltage applied at the base of the transistor.

But I obviously make a mistake because it simply doesn`t work at all!

post-15243-14279143088667_thumb.jpg

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OK Nick, I did misunderstand your question!


It`s ok! Perhaps I didn`t express myself very clearly and I should have posted that picture too!

Well I simply triggered it by connecting it to the ground or to the positive supply (later at my real circuit this will be connected to the output port of a PIC, but I guess it is the same thing!)

I don`t know, I may have made a mistake... I will check it once more!
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Thanks a lot Zeppelin! That resistor must be my mistake (I had thought of it, but couldn`t find the proper value). The guide is very helpful on that too!


With a 5V supply you can't drive that many LEDs, tops 3, I guess-you need a higher supply voltage.


I know that, I intend to use only two LEDs on series and organize all of them on doubles!
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